Objective: The long-term goal of this research project is to contribute to a full understanding of the molecular and biochemical mechanisms underlying the antitumor action of 5-fluorouracil (FU) and related antimetabolites. One very important aim of this overall goal is to learn more about the metabolic pathways by which FU is converted into its therapeutically-active nucleotide derivatives. Approach: Experiments outlined in this proposal will explore the basic molecular properties of a pyrimidine phosphoribosyltransferase of potentially important cancer chemotherapeutic significance. This enzyme, which we have found to be present in a wide array of transplantable murine leukemias, not only utilizes FU as a substrate but its activity serves as a major determinant of FU sensitivity in murine leukemias. Plans call for this enzyme to be isolated and characterized from murine leukemias of known and different sensitivity to FU. This comparative enzymology study is expected to provide new insight into the molecular basis of tumor sensitivity to FU. Characterization of the phosphoribosyltransferase from a variety of normal tissues may yield data of a clinically exploitable nature. That is, any fundamental differences in the molecular properties of this enzyme from normal and leukemic cells may provide a basis for the rational design of new FU-like drugs having increased potency and tumor selectivity.